Synchronization deviation detection

ABSTRACT

A method for monitoring synchronization deviation in a mobile telecommunication system is disclosed. The system records the deviation in the synchronization between base stations which may be computed from measurements made at handover of a mobile station between base stations and when the deviation exceeds a predetermined threshold, an alarm is triggered which provides an alert to adjust the synchronization of the system.

BACKGROUND

The present invention relates to a method and apparatus for monitoringsynchronization between base stations in a telecommunications systemand, more specifically, to a method and apparatus for detectingsynchronization deviation between the base stations.

In a cellular telecommunication system such as cellular telephone system100 illustrated in FIG. 1, there are a plurality of cells, for example,cells C₀-C₉. Typically, each of the cells C₀-C₉ has a single basestation, for example, base stations B₀-B₉. In general, each base stationB₀-B₉ transmits and receives communication signals, using a basetransceiver station (BTS), to and from mobile units within thecorresponding cell. Therefore, the system of FIG. 1 includes BTS₀-BTS₉corresponding to base stations B₀-B₉.

When a mobile unit such as M₄ moves from a cell, C₀, covered by a firstbase station transceiver BTS₀ towards another cell, C₁, covered by asecond base station transceiver BTS₁, the responsibility forcommunication with the mobile unit M₄ must be transferred from BTS₀ toBTS₁. Such transfer in conjunction with an established connection to themobile station is usually called a “handover”.

Handover requires careful system management to reduce the risk of theestablished connection being temporarily interrupted or completely lostduring the handover. Such interruption and/or loss of service may resultfrom, among other things, lack of synchronization between referencetiming sources employed by the first base station and the second basestation. Therefore, maintaining synchronization between base stations incellular telecommunication system is extremely important.

The BTS uses an external or internal reference timing source to generateaccurate radio carrier frequencies (i.e., such as one having an absolutefault of approximately 0.1 parts per million [ppm]) as well as togenerate accurate timing of the bit stream transmitted over the radiointerface. In instances where the BTS uses an external source as atiming reference synchronization, the clock source is located in thepublic switching telephone network (PSTN). The BTS synchronization isthen taken from the PSTN via the mobile switching center (MSC), the basestation controller (BSC) and transmission networks. Outside the UnitedStates, a common timing reference is used for each BTS in a network. Ifthe BTS uses an internal clock source on the other hand, then thissource is usually a high stability oscillator. In the United States,each BTS has a local (internal) source.

If the timing reference employed by a BTS is unreliable, a mobilestation attempting to synchronize itself to the BTS may encounterdifficulties. In extreme cases, the mobile unit may not be able toaccess the BTS. For example, referring again to FIG. 1, consider thatmobile unit M₄ has a call set-up using BTS₀ corresponding to basestation B₀. BTS₀ is synchronized to the PSTN and M₄ is synchronized toBTS₀. Mobile station M₄ will typically be requested to measure andreport signal strength associated with neighboring base stations'transmission for the purpose of identifying when handover to anotherbase station is desirable. As M₄ moves towards BTS₁, if the timingreference employed by BTS₁ is not reliable and, therefore, BTS₁ istransmitting information at a different frequency and/or with adifferent timing than that expected by M₄, then M₄ may not be able toread the base station identity code (BSIC) associated with BTS₁. In sucha situation, M₄ will not report BTS₁ as a handover candidate to BTS₀which is currently in communication with M₄. If BTS₁ is not synchronizedwith the PSTN, the call will drop after M₄ enters an area where onlyBTS₁ can provide communication service support.

Unreliable sources of synchronization can cause frequency drift of thebase stations over a period of time. These sources are one reason thatcause calls to be dropped, lost or interrupted. Other reasons mayinclude unsynchronized clocks, faulty configuration of intermediatenetwork modules such as DXXs, DXCs and BSCs as well as BTS faults andlink breaks.

The synchronization problems usually come to the attention of systemoperators via, among other means, end user (i.e., subscriber) complaintsof dropped calls. As a result, identifying dropped calls that resultfrom lack of synchronization is a time consuming process. Identifying asynchronization problem from statistical data is also a very slow andunreliable process, for example, in cases where the mobile unit of aparticular brand is unable to read a base station identity code (BSIC)from a neighboring cell. Furthermore, if the synchronization deviationis changing slowly, the detection of this problem becomes harder.

What is desired, therefore, is a method and apparatus for detectingsynchronization deviation in a cellular telecommunications system.

SUMMARY

It is an object of the present invention to overcome the deficienciesdescribed above by providing a method for utilizing existingfunctionality of a mobile unit for monitoring synchronization deviationin a cellular telecommunications system.

According to exemplary embodiments of the present invention, a systemfor monitoring synchronization in a radio telecommunication system isdisclosed which comprises a first and a second base station each havinga transceiver associated therewith; at least one mobile station having atransceiver and communicating with the first base station prior to beinghanded over to the second base station; a processor, associated with theat least one mobile station for determining a time difference valuebetween a signal received from the first base station and a signalreceived from the second base station, wherein the mobile stationtransmits the time difference value to the system; and a controller,within the system, for receiving said time difference value anddetermining whether the first and second base stations are synchronizedbased on the time difference value.

Other embodiments of the present invention disclose a method formaintaining synchronization in a radio telecommunication system having aplurality of base stations and a plurality of mobile stations. Themethod comprises the steps of monitoring synchronization deviationbetween at least one pair of base stations within the plurality of basestations, wherein a deviation in the synchronization is computed by abase station controller (BSC) based on values reported to the BSC by amobile station being handed over from a first one of the pair of basestations to a second one of the pair of base stations and on a valuereported to the BSC by a base station transceiver associated with thesecond of said base stations; and triggering an alarm if the monitoringindicates that a predetermined threshold for a deviation in thesynchronization has been exceeded.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects, features and advantages of the presentinvention will be readily apparent to one skilled in the art from thefollowing written description, read in conjunction with the drawings, inwhich:

FIG. 1 illustrates a conventional cellular telecommunicationscommunications network;

FIG. 2 illustrates a cellular communications telecommunications networkfor practicing exemplary embodiments of the present invention; and

FIG. 3 illustrates a method for monitoring synchronization in a cellulartelecommunication network according to exemplary embodiments of thepresent invention.

DETAILED DESCRIPTION

In the following description, for purposes of explanation and notlimitation, specific details are set forth, such as particular circuits,circuit components, techniques, etc. in order to provide a thoroughunderstanding of the present invention. However, it will be apparent toone skilled in the art that the present invention may be practiced inother embodiments that depart from these specific details. In otherinstances, detailed descriptions of well-known methods, devices, andcircuits are omitted so as not to obscure the description of the presentinvention.

The exemplary radio communication systems discussed herein are describedas using the time division multiple access (TDMA) protocol, in whichcommunication between the base station and the mobile terminals isperformed over a number of time slots. However, those skilled in the artwill appreciate that the concepts disclosed herein find use in otherprotocols, including, but not limited to, frequency division multipleaccess (FDMA), code division multiple access (CDMA), or some hybrid ofany of the above protocols. Likewise, some of the exemplary embodimentsprovide illustrative examples relating to the GSM system, however, thetechniques described herein are equally applicable to radio basestations in any system.

A mobile station may be able to receive signals from a plurality of basestations. Upon instruction from the system and when the mobile stationis not otherwise occupied e.g., transmitting or receiving, it can thenmeasure the signal strength of these received signals and communicatethe measured values to a base station controller (BSC). The BSC usesthese measurements to identify base stations as candidates for handoverwhen the mobile station moves away from the base station that itcurrently is in communication with. The signals received by the mobilestation from the various base stations may also enable the mobilestation to, among other things, determine the relative timesynchronization (e.g., frame sync) between the transmissions of theplurality of base stations from which the mobile station is capable ofreceiving signals. Thus, in accordance with exemplary embodiments of thepresent invention, signals received by a mobile may be used to determinethe relative frame offset between base stations, as well as for makingsignal strength measurements.

A general description and definition of the various values used incomputing the frame synchronization is included here for a clearunderstanding of exemplary embodiments of the present invention. AllBTSs within a system are synchronized at one time, such as when thenetwork is established, and subsequently become unsynchronized. UnderGSM, when a mobile station (MS) is to perform a handover from BTS₀(current or old base transceiver station) to BTS₁ (or new basetransceiver station), the following quantities are defined. The line ofsight propagation delay between MS and BTS₀ is denoted as t₀; similarlythe line of sight propagation delay between MS and BTS₁ is denoted ast₁. The difference in the local system time between BTS₀ and BTS₁ isdenoted as RTD (real time difference or, also referred to as a frameoffset value). The timing difference between BTS₀ and BTS₁ as measuredby the MS is denoted as OTD (or, observed time difference). These valuesmay all be varying over time due to, for example, movement of the MS anddrift in oscillators associated with BTSs. The following equationdescribes the relationship between the various values listed above:

OTD=RTD+t ₁ −t ₀  (1)

A handover may be synchronized, pseudo synchronized or unsynchronized.The difference between a synchronized and a pseudo synchronized handovermay be described as follows. In a synchronized handover, both BTSs(i.e., the BTSs involved in the handover) send the TDMA frame structureat the same time which means that the RTD real time difference is always0 (frame offset=0). In a pseudo synchronized handover, the RTD (frameoffset) between the BTSs is known by the system and the RTD is sent tothe MS in a handover command message. Synchronized andpseudo-synchronized handovers use one signaling sequence where normalsized bursts are being used during the entire handover sequence.

The unsynchronized handover uses a slightly different and a longersignaling sequence which also contains a time alignment procedure wherethe mobile station starts to send several short bursts which will bereceived within a much bigger (i.e., a longer time) time slot. The BTSwill measure where in the time slot the burst is received. The BTS willthen inform the mobile station, in a info message, of what timingadvance to use. That is, the BTS “tells” the mobile station how muchearlier the mobile station should send the next normal sized burst.

In a synchronized handover of a MS to BTS₁ ordered by BTS₀, the MS setsthe RTD value to zero (i.e., RTD=0) since the base stations(corresponding to BTS₀ and BTS₁) are synchronized. In a synchronizedhandover, all base stations send the frames at exactly the same time.Under normal conditions, since the mobile station is alreadysynchronized to BTS₁ with which it is currently communicating via atraffic channel before the handover is executed, values for OTD, RTD andt₀ are available to MS. Therefore, the value for t₁ can be easilycomputed by MS without the MS having to rely on BTS₁ for providing thisvalue. Using the equation above,

t ₁ =OTD−RTD+t ₀  (2)

Upon completion of a handover, the MS may provide to BTS₁ the value ofOTD+t₀ in the handover complete message. This value enables BTS₁ toobtain an estimate of RTD. In estimating the RTD, BTS₁ takes intoconsideration its estimate of t₁. In order to mitigate the effects ofestimation errors and quantization effects however, additionalprocessing may be needed for determining these values.

In a pseudo synchronized handover, it is assumed that the RTD is knownto BTS₀ and that the mobile station supports this convention. When BTS₀orders a psuedo-synchronized handover of a MS to BTS₁, BTS₀ reports theknown RTD value to this MS. The RTD value is sent to the mobile stationin a handover command. The mobile station then knows exactly when tosend the first normal sized burst to BTS₁.

As the present invention is primarily concerned with lack ofsynchronization, the description of computations for the various valueswill now be described for a handover in which BTS₀ and BTS₁ are notsynchronized with each other. It should be pointed that the methods ofthe present invention are not limited to an unsynchronized system. Inthe unsynchronized case, the MS may be ordered to report a TimeDifference Value (TDV) which is the sum of OTD and to (i.e., TDV=OTD+t₀)to the base station controller (BSC) in a handover complete message. Thetarget base transceiver station (BTS₁ in this case) may be programmed toreport t₁ to the BSC. Based on these received values, the BSC cancompute the RTD by using the equations given above which may also beexpressed as:

RTD=OTD+t ₀ −t ₁  (3)

which may also be expressed as

RTD=TDV−t ₁  (4)

As described above, the RTD value is currently used for providingsynchronization information to the mobile station in apseudo-synchronized handover. It is expressed in half-bit periods eachof which is, according to the GSM standard, 24/13 micro seconds. Thecomputation for the RTD is performed in the BSC. In the unsynchronizedcase, the MS has to transmit very short burst to BTS₁ at handover. BTS₁will then measure the propagation delay to the MS and reports it to theMS in the next message. The MS can then use normal sized bursts.

According to exemplary embodiments of the present invention, the system,e.g., the BSC may maintain a record of the computed frame offset values(or, RTD values) for each pair of base stations in the mobilecommunications network. In the system of FIG. 1, for instance, a recordof the frame offset values may be maintained for handovers between BTS₀and each of BTS₁ to BTS₉; similarly, a record of frame offset values maybe maintained for handovers between BTS₁ and each of BTS₂ to BTS₉. Inaddition, the BSC may compute a coefficient k based on the timedifference measured by the mobile stations, which coefficient representsthe synchronization deviation using the following equations:

k _(m) =P _(m)/Q _(m)  (5)

where $\begin{matrix}{P_{m} = {{\sum\limits_{i = {m - n}}^{m}\quad {t_{i}\quad s_{i}}} - {{1/n}\quad \left( {\sum\limits_{i = {m - n}}^{m}\quad t_{i}} \right)\quad \left( {\sum\limits_{i = {m - n}}^{m}\quad s_{i}} \right)\quad {and}}}} & (6) \\{Q_{m} = {{\sum\limits_{i = {m - n}}^{m}\quad t_{i}^{2}} - {{1/n}\quad \left( {\sum\limits_{i = {m - n}}^{m}\quad t_{i}} \right)^{2}}}} & (7)\end{matrix}$

where s_(i) is RTD for sample i (i.e., handover number i), t_(i) is thetime at which RTD for sample i was measured, n is the number of samplesand m is the number of the last sample. In order for the value of k tobe valid, m must be greater than or equal to n.

A k is computed and maintained for each pair of base stations in thesystem. If the system consists of three base stations, X, Y and Z forexample, then a k value may be computed and maintained for handoversbetween X and Y; another k value may be computed and maintained forhandovers between Y and Z and a third k value may be computed andmaintained for handovers between X and Z. The coefficient k may bemonitored by the system to detect when the time deviation has reached anunacceptable level. An unacceptable level may be that at which the lackof synchronization within the network has reached a level at whichhandover becomes (or is about to become) difficult and breaks incommunication will result if the deviation is allowed to increase. Forexample, the absolute value of k_(m) may be used to monitor thedeviation in the frame synchronization. If k_(m) exceeds a maximumallowed deviation value, an alarm may be triggered. This may beexpressed as:

if |k _(m) |> X, then Y

where X is the maximum allowed deviation and Y is an alarm beingtriggered.

The alarm may prompt an operator to intervene and adjust thesynchronization among the base stations. In response, the operator may,among other things, check the transport network, adjust the clock orchange the clock.

Exemplary embodiments of the present invention may be more clearlyunderstood with reference to FIG. 2 which highlights a cellulartelecommunications network with a storage device at the base stationcontroller (BSC) for storing the various values reported to the BSC andvalues computed by the BSC.

Upon the completion of a handover from an originating base stationtransceiver 205 (BTS₀) to a destination base station transceiver 210(BTS₁) within the cellular system 200, a time difference value between,for example, the timing of the frame structure transmitted by BS₀ andthat transmitted by BS₁ (i.e., TDV which is the same as OTD+t₀) isdetermined by the mobile station 220 (MS) which has been handed over.This value can be communicated by the MS to the base station controller225 (BSC). In addition, BTS₁ (the destination BTS) reports a t₁ value tothe BSC. Each of B₀ and B₁ may represent any of the base stations in thecellular system as long as B₀ and B₁ are not the same base station. Thisinformation (i.e., OTD+t₀ and t₁) enables the BSC to compute not onlythe frame offset value, RTD, but also the coefficient k used to monitorthe synchronization deviation. These values may be recorded in a storagedevice 230, e.g., RAM or secondary storage, associated with the BSC 225.

The coefficient k may be monitored as described above and an alarm 240may be triggered if the synchronization deviation exceeds apredetermined threshold. The threshold may vary from system to systembased on such variables as, for example, the number of base stations inthe cellular system and traffic volume within the system. An exemplarymethod for performing the steps of the present invention for monitoringsynchronization in a cellular telecommunication system may beillustrated with reference to FIG. 3.

In a cellular telecommunication system, a handover of a mobile stationfrom a first base station to a second base station is initiated at 302.As part of the completion of the handover, the mobile station determinesa time difference value (TDV) between transmissions received from thefirst and second base stations at 304. This value is communicated by themobile station to the base station controller (BSC) at 306. Thedestination BTS (BTS₁) reports a t₁ value to the BSC at 308. The BSCcomputes a frame offset value (RTD) based on the reported TDV and t₁ andthe deviation value at 310. These values are stored in the BSC at 312 ina location which corresponds to the pair of base stations involved inthe handover at 302. This process is repeated for each handover that iscompleted.

In addition, as k is computed upon the completion of a handover betweena pair of base stations, it is compared with a predetermined threshold(TH) value at 314. If the threshold value is exceeded, an alarm istriggered at 316 which results in an operator readjusting the system at318. If k does not exceed the predetermined threshold at 314, theprocess of measuring, reporting and computing of values is repeated.Similarly, after the system is synchronized at 318, the above mentionedfunctions are repeated.

It should be noted that exemplary methods of the present invention arenot limited to application in unsynchronized networks. It could also beused in networks having synchronized handovers. As mentioned above, thenetwork is synchronized at one time. The present invention has beendescribed in terms of specific embodiments to facilitate understanding.The above embodiments, however, are illustrative rather thanrestrictive. It will be readily apparent to one skilled in the art thatdepartures may be made from the specific embodiments shown above withoutdeparting from the central spirit and scope of the invention. Therefore,the invention should not be regarded as being limited to the aboveexamples, but should be regarded instead as being fully commensurate inscope with the following claims.

What is claimed is:
 1. A system for detecting synchronization deviationin a radio telecommunication system comprising: a first and a secondbase station each having a transceiver associated therewith; at leastone mobile station having a transceiver and communicating with saidfirst base station prior to being handed over to the second basestation; a processor, associated with said at least one mobile stationfor determining a time difference value between a signal received fromsaid first base station and a signal received from said second basestation, wherein said mobile station transmits said time differencevalue to said system; and a controller, within said system, forreceiving said time difference value and determining whether asynchronization deviation has occurred between said first and secondbase stations based on said time difference value.
 2. The system ofclaim 1, wherein the controller comprises: a processing means forcomputing a frame offset value from a plurality of received timedifference values; and a comparing means for comparing the frame offsetvalue with a predetermined threshold.
 3. The system of claim 2, whereinthe controller further comprises a storage means for storing thecomputed frame offset value.
 4. The system of claim 2, wherein thecontroller further comprises an alarm means for triggering an alert whenthe comparing means indicates that the predetermined threshold has beenexceeded, the alert indicating that the base stations within the systemare not synchronized.
 5. The system of claim 4, wherein the controllerfurther comprises a correction means for correcting the synchronization.6. The system of claim 4, wherein an operator synchronizes the basestations upon the triggering of the alert.
 7. The system of claim 6,wherein the synchronization of the base stations comprises adjusting aclock of a base station transceiver.
 8. The system of claim 6, whereinthe synchronization of the base stations comprises adjusting a clocksource which provides timing reference to the base station transceivers.9. A method for maintaining synchronization in a radio telecommunicationsystem having a plurality of base stations and a plurality of mobilestations, said method comprising the steps of: monitoringsynchronization deviation between at least one pair of base stationswithin said plurality of base stations, wherein a deviation in saidsynchronization is computed by a base station controller (BSC) based onat least one value reported to the BSC by a mobile station being handedover from a first one of said pair of base stations to a second one ofsaid pair of base stations and on at least one value reported to the BSCby a base station transceiver associated with the second of said basestations; and triggering an alarm if the monitoring indicates that apredetermined threshold for a deviation in said synchronization has beenexceeded.
 10. The method of claim 9, further comprising the steps of:communicating a time difference value by the mobile station to the BSC;and communicating a timing advance value by the base station transceiverof the second base station to the BSC.
 11. The method of claim 10,further comprising the steps of: computing, by said BSC, a coefficientrepresenting a deviation in synchronization, said coefficient beingcomputed from the values received by the BSC.
 12. The method of claim11, further comprising the steps of: comparing the coefficient to apredetermined threshold; and triggering an alarm if the coefficientexceeds the threshold.
 13. The method of claim 12, further comprisingthe step of prompting an operator to readjust the synchronization in thenetwork.
 14. The method of claim 12, wherein the readjustment ofsynchronization comprises adjustment of a clock of a base stationtransceiver.